Communication terminal having means for controlling power transfer

ABSTRACT

A communication terminal is coupled to a communication network via transmitting and receiving signal lines and a switching system. The communication terminal includes a power input line coupled to the transmitting and receiving signal lines, a power sink for supplying power, a switch coupled between the power input line and the power sink, a hook switch coupled between the power input line and the power sink in parallel to the switch, a detection circuit coupled to the receiving signal line for detecting a signal on the receiving signal line, a state maintaining circuit coupled to the switch and the detection circuit for closing the switch and for maintaining the switch in a closed state by carrying out a state maintaining operation when a signal is detected by the detection circuit, and a prohibit circuit coupled to the detection circuit for prohibiting a signal detection of the detection circuit.

BACKGROUND OF THE INVENTION

The present invention generally relates to communication terminals, andmore particularly to a communication terminal which controls powertransfer and is suited for application to an integrated services digitalnetwork (ISDN) terminal.

An ISDN provides a plurality of different communication services such astelephone, data, facsimile and image communications by use of acommunication network including a digital switching system and digitaltransmission lines. In the ISDN, when a power down occurs on the network(switching system) side, restricted power with a polarity which isinverted from a normal power is supplied from the network side.

FIG. 1 shows an example of a conventional ISDN. A plurality of ISDNterminals TE are coupled to an ISDN switching system EX via networkterminator devices (or digital service units) NT1 and NT2. The networkterminator device NT1 and NT2 include functions for making subscriber'sline termination, layer 1 maintenance, monitoring, power transfer, layer1 multiplexing, interface termination, protocol processes of layers 2and 3, layer 2 multiplexing, layer 3 multiplexing, and interfacetermination.

FIG. 2 shows an example of a conventional ISDN terminal. A transmissionsignal line 100 is coupled to a frame disassembly/assembly part 105 of aline interface part 104 via a transformer T1 and drivers 102. Areceiving signal line 101 is coupled to the frame disassembly/assemblypart 105 of the line interface part 104 via a transformer T2 andreceivers 103. The frame disassembly/assembly part 105disassembles/assembles the B1 and B2 channels and the D channel within2B+D. The D channel is transmitted and received between a centralprocessing unit (CPU) interface 106 of the line interface part 104 andeach of a CPU 108, a random access memory (RAM) 109 and a read onlymemory (ROM) 110 via a data bus 107. The B1 and B2 channels aretransmitted and received between a B channel selector 111 of the lineinterface part 104 and a handset 115 via a coder/decoder 112, a sendingamplifier 113 and a receiving amplifier 114. A dialing part 119 isconnected to the CPU 108.

On the other hand, the signal lines 100 and 101 are coupled to a powerinput line 117 via a diode bridge 116, and the power input line 117 iscoupled to internal circuits via a power sink 118 and a power input line219. The power is supplied to the power sink 118 regardless of whetheror not the ISDN terminal is carrying out a communication.

For example, there are cases where a plurality of terminals having nolocal power supply are connected to the communication network via theswitching system and cases where the local power supply is down. In suchcases, when the plurality of terminals simultaneously require the powersupply from the switching system side, the switching system inconformance with a communication standard can only supply 400 mW persubscriber's line. As a result, there are problems in that the load onthe network side increases and the terminal side cannot operate in somecases. In other words, according to this communication standard, oneterminal is only supplied with a power of 380 mW from the switchingsystem side so that the remaining 20 mW can be supplied to otherterminals to enable at least a terminal to detect a call. However, sincethe terminal supplied with the power of 380 mW constantly operates adetection circuit for detecting the call, considerable power is consumedby the detection circuit and the power supplied to internal circuits ofthis terminal is reduced thereby. Therefore, there is a demand to fullyutilize the limited power supplied to the terminal from the switchingsystem side.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful communication terminal in which the problemsdescribed above are eliminated.

Another and more specific object of the present invention is to providea communication terminal which is coupled to a communication network viatransmitting and receiving signal lines and a switching system,comprising a power input line coupled to the transmitting and receivingsignal lines, a power sink for supplying power, switching means coupledbetween the power input line and the power sink, a hook switch coupledbetween the power input line and the power sink in parallel to theswitching means, detection circuit means coupled to the receiving signalline for detecting a signal on the receiving signal line, statemaintaining circuit means coupled to the switching means and thedetection circuit means for closing the switching means and formaintaining the switching means in a closed state by carrying out astate maintaining operation when a signal is detected by the detectioncircuit means, and prohibit circuit means coupled to the detectioncircuit means for prohibiting a signal detection of the detectioncircuit means. According to the communication terminal of the presentinvention, it is possible to suppress the power consumption to a minimumwhen a power down occurs, so as to reduce the load on the network(switching system) side. In addition, it is possible to reduce thecapacity of the power source on the switching system side.

Still another object of the present invention is to provide acommunication terminal which is coupled to a communication network viatransmitting and receiving signal lines and a switching system,comprising a power input line coupled to the transmitting and receivingsignal lines, a power sink coupled to the power input line for supplyingpower from an output side thereof, switching means coupled to the outputside of the power sink, a hook switch coupled to the output side of thepower sink in parallel to the switching means, detection circuit meanscoupled to the receiving signal line for detecting a signal on thereceiving signal line, state maintaining circuit means coupled to theswitching means and the detection circuit means for closing theswitching means and for maintaining the switching means in a closedstate by carrying out a state maintaining operation when a signal isdetected by the detection circuit means, and prohibit circuit meanscoupled to the detection circuit means for prohibiting a signaldetection of the detection circuit means. According to the communicationterminal of the present invention, it is possible to suppress the powerconsumption to a minimum when a power down occurs, so as to reduce theload on the network (switching system) side. In addition, it is possibleto reduce the capacity of the power source on the switching system side.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram showing an example of a conventionalISDN;

FIG. 2 is a system block diagram showing an example of a conventionalISDN terminal;

FIG. 3 is a system block diagram for explaining an operating principleof a communication terminal according to the present invention;

FIG. 4 is a system block diagram showing a first embodiment of thecommunication terminal according to the present invention;

FIG. 5 is a timing chart showing various timings when the ISDN terminalis called;

FIG. 6 shows a sequence of operations which are carried out when theISDN terminal is called;

FIG. 7 is a timing chart showing various timings when the ISDN terminalis calling;

FIG. 8 shows a sequence of operations which are carried out when theISDN terminal is calling;

FIG. 9 is a timing chart showing various timings when the disconnectionis made from the ISDN side;

FIG. 10 shows a sequence of operations which are carried out when thedisconnection is made from the ISDN side;

FIG. 11 is a timing chart showing various timings when the disconnectionis made from the ISDN terminal side;

FIG. 12 shows a sequence of operations which are carried out when thedisconnection is made from the ISDN terminal side;

FIG. 13 shows the structure of one frame of layer 1;

FIG. 14 is a diagram for explaining the definition of the signal Info;

FIG. 15 is a circuit diagram showing another embodiment of the statemaintaining circuit; and

FIG. 16 is a system block diagram showing a second embodiment of thecommunication terminal according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a description will be given of an operating principle of acommunication terminal according to the present invention, by referringto FIG. 3. In FIG. 3, those parts which are basically the same as thosecorresponding parts in FIG. 2 are designated by the same referencenumerals.

In FIG. 3, a switch SW is connected in parallel to a hook switch HS1.The signal line 100 is coupled to the line interface part 104 via thetransformer T1 and the drivers 102, and the signal line 101 is coupledto the line interface part 104 via the transformer T2 and the receivers103. These signal lines 100 and 101 couple the communication terminal toa communication network (not shown) via a switching system (not shown).The signal lines 100 and 101 are also coupled to the power input line117 via the diode bridge 116 and parallel connection of the switches SWand HS1, and the power input line 117 is connected to the power sink118. The power sink 118 is used when the power is down on thecommunication network side and supplies the power via the power inputline 219.

A line signal detection circuit 1 detects a signal on the signal line101. A state maintaining circuit 2 closes the switch SW and carries outa maintaining operation to maintain the switch SW in the closed statewhen the line signal detection circuit 1 detects a signal on the signalline 101. As a result, power is supplied from the switching system tothe power sink 118 via the power input line 118. A prohibit circuit 3prohibits the line signal detection of the line signal detection circuit1 in response to the closing operation cf the hook switch HS1. A cancelcircuit 4 cancels the maintaining operation of the state maintainingcircuit 2 in response to a signal which is received from a controller 5when the hook switch HS1 is open.

Next, a description will be given of a first embodiment of thecommunication terminal according to the present invention, by referringto FIG. 4. In this embodiment, the present invention is applied to anISDN terminal. In FIG. 4, those parts which are basically the same asthose corresponding parts in FIG. 3 are designated by the same referencenumerals.

A switching transistor Tr1 is connected in parallel to the hook switchHS1. The signal line 100 is coupled to the line interface part 104 viathe transformer T1 and the drivers 102, and the signal line 101 iscoupled to the line interface part 104 via the transformer T2 and thereceivers 103. These signal lines 100 and 101 couple the ISDN terminalto an ISDN (not shown) via a switching system (not shown). The signallines 100 and 101 are also coupled to the power input line 117 via thediode bridge 116 and the parallel connection of the hook switch HS1 andthe switching transistor Tr1, and the power input line 117 is connectedto the power sink 118. The diode bridge 116 includes diodes D6 throughD9. For example, a semiconductor switching element such as a fieldeffect transistor (FET) may be used for the switching transistor Tr1.

The line interface part 104 has the same construction as the lineinterface part 104 shown in FIG. 2. In other words, the line interfacepart 104 has a frame disassembly/assembly part, a CPU interface and a Bchannel selector. The D channel is transmitted and received between theCPU interface of the line interface part 104 and each of a CPU 108, aRAM and a ROM via a data bus. The B1 and B2 channels are transmitted andreceived between the B channel selector of the line interface part 104and a handset via a coder/decoder, a sending amplifier and a receivingamplifier.

As described above, the signal detection circuit 1 detects the signal onthe signal line 101. The line signal detection circuit 1 shown in FIG. 4includes diodes D1 through D4, resistors r1 through r6 and r8,capacitors C1 through C4 and a comparator CP1 which are connected asshown. The level of the input signal on the signal line 101 is amplifiedto approximately two times by the capacitors C3 and C4, and the signalon the signal line 101 is detected by the comparator CP1 which uses athreshold voltage V_(TH) which is a divided voltage determined by theresistors r5 and r6.

When the line signal detection circuit 1 detects the signal on thesignal line 101, the state maintaining circuit 2 turns the switchingtransistor Tr1 ON and maintains the switching transistor Tr1 in the ONstate. The state maintaining circuit 2 shown in FIG. 4 includes a diodeD5, resistors r9 through r11, a capacitor C5, and a transistor Tr2. Whenthe line signal detection circuit 1 detects the signal on the signalline 101, the capacitor C5 is charged and the transistor Tr2 turns ON,thereby turning the switching transistor Tr1 ON. As a result, the poweris supplied to the power sink 11B so as to supply the power to theinternal circuits. The resistor r11 is provided in order to prevent theswitching transistor Tr1 from turning OFF when the level of the signalon the signal line 101 becomes small. As a result, the transistors Tr2and Tr1 are maintained in the ON state.

As described above, the prohibit circuit 3 prohibits the line signaldetection of the like signal detection circuit 1 in response to theclosing operation of the hook switch HS1. The prohibit circuit 3includes a sub hook switch HS2 which is linked to the hook switch HS1and opens in response to the closing operation of the hook switch HS1.Hence, the supply of power to the comparator CP1 is cut off when the subhook switch HS2 is open and the loss of power to the power sink 118 issuppressed to a minimum. In this state, the line signal detectioncircuit 1 does not operate, and there is absolutely no power loss at theline signal detection circuit 1.

The cancel circuit 4 cancels the maintaining operation of the statemaintaining circuit 2 in response to a signal which is received from theCPU (controller) 5 when the hook switch HS1 is open. The CPU 5 controlsthe total operation of the ISDN terminal. The cancel circuit 4 includesa photocoupler PC1. After the communication state ends or when anotherterminal is selected after the line signal detection, the photocouplerPC1 turns OFF (that is, disables) the state maintaining circuit 2 inresponse to the signal from the CPU 5. Particularly, after thecommunication state ends or when another terminal is selected after theline signal detection, the CPU 5 after ending all other processessupplies a switch off signal to the photocoupler PC1 to turn thephotocoupler PC1 ON. This switch off signal has a voltage V_(SW). As aresult, the voltage V_(C) stored in the capacitor C5 is discharged andthe transistor Tr2 turns OFF. At the same time, the switching transistorTr1 turns OFF and the state maintaining circuit 2 turns OFF. As aresult, the supply of power to the internal circuits is stopped and thepower consumption is suppressed to a minimum during states other thanthe communication state.

The four basic operations of the ISDN terminal will now be described.

1) Operation When Called:

The capacitances of the capacitors C1 and C2 and the resistances of theresistors r1 and r2 are selected to high impedances so as not to affectthe input impedance of the receiver. The capacitors C1 and C2 areprovided to cut the D.C. component, and the diodes D1 through D4 rectifythe line signal when the receiver is called. The capacitors C3 and C4are respectively charged with a time constant determined by the resistorr1 and the capacitor C3 and a time constant determined by the resistorr2 and the capacitor C4. The input voltage is amplified to approximatelytwo times by the capacitors C3 and C4 and is supplied to the comparatorCP1 as a line input voltage V_(L). The threshold voltage V_(TH) isdetermined by the resistors r5 and r6 and is supplied to the comparatorCP1. When the line input voltage V_(L) exceeds the threshold voltageV_(TH), the comparator CP1 outputs an output voltage V_(CO).

When the comparator CP1 outputs the output voltage V_(CO), the capacitorC5 is charged with a time constant determined by the resistors r8 andr12. When the voltage V_(C) of the capacitor C5 reaches a predeterminedvoltage level, the transistor Tr2 turns ON, and at the same time, theswitching transistor Tr1 turns ON. When the switching transistor Tr1turns ON, a power sink input voltage V_(I) is supplied to the power sink118. In this state, the resistor r11, the transistor Tr2 and the diodeD5 cooperate to maintain the ON state of the switching transistor Tr1.Hence, even when the signal on the signal line 101 becomes small and thecomparator CP1 turns OFF, the switching transistor Tr1 is maintained inthe ON state and the power sink input voltage V_(I) continues to besupplied to the power sink 118. For this reason, stable power issupplied to the internal circuits. The power sink 118 supplies a powersink output voltage V_(O) to the internal circuits.

Because the switch off signal (voltage V_(SW)) from the CPU 5 has a highlevel in this state, the photocoupler PC1 remains OFF, that is, will notoperate.

Thereafter, the layer 1 is started and it becomes possible to make acommunication. When the hook switch HS1 turns ON responsive to anoff-hook, the sub hook switch HS2 turns OFF and the line signaldetection circuit 1 is turned OFF. As a result, it is possible to keepthe power supply to a minimum in the communication state.

FIG. 5 is a timing chart showing the various timings when the ISDNterminal is called, and FIG. 6 shows a sequence of operations which arecarried out when the ISDN terminal is called.

FIG. 5(A) shows the signal received on the signal line 101 and FIG. 5(B)shows the signal transmitted on the signal line 100. FIG. 5(C) shows theline input voltage V_(L) and the threshold voltage V_(TH) which aresupplied to the comparator CP1, and FIG. 5(D) shows the output voltageV_(CO) of the comparator CP1. FIG. 5(E) shows the voltage V_(C) acrossthe capacitor C5. FIG. 5(F) shows the ON/OFF state of the switchingtransistor Tr1, and FIG. 5(G) shows the ON/OFF state of the transistorTr2. FIG. 5(H) shows the power sink input voltage V_(I) supplied to thepower sink 118, FIG. 5(I) shows the power sink output voltage V_(O)output from the power sink 118, and FIG. 5(J) shows the voltage V_(SW)output from the CPU 5. FIG. 5(K) shows the ON/OFF state of the hookswitch HS1, and FIG. 5(L) shows the ON/OFF state of the sub hook switchHS2.

In FIG. 6, the ISDN supplies the signal Info0 to the ISDN terminal andthe ISDN terminal supplies the signal Info0 to the ISDN. In this state,the power is OFF. Then the ISDN supplies the signal Info2 to the ISDNterminal and the power turns ON. The ISDN terminal supplies the signalInfo3 in response to the signal Info2. Thereafter, the ISDN supplies thesignal Info4 to the ISDN terminal and makes a set up with respect to theISDN terminal. The ISDN terminal makes an alerting and a connect by anoff-hook. The ISDN makes a connect acknowledge and a communication stateis reached between the ISDN and the ISDN terminal.

2) Operation When Calling:

In this case, the hook switch HS1 turns ON and the sub hook switch HS2turns OFF responsive to an off-hook. Hence the line signal detectioncircuit 1 is turned OFF and it is possible to keep the power supply to aminimum. In addition, the state maintaining circuit 2 operates similarlyas in the case of the called state described above to maintain the ONstate of the switching transistor Tr1, and the photocoupler PC1 of thecancel circuit 4 is also OFF.

Accordingly, the voltages V_(I) and V_(O) are successively supplied andthe layer 1 is started so that it becomes possible to make acommunication.

FIG. 7 is a timing chart showing the various timings when the ISDNterminal is calling, and FIG. 8 shows a sequence of operations which arecarried out when the ISDN terminal is calling.

FIG. 7(A) shows the signal received on the signal line 101 and FIG. 7(B)shows the signal transmitted on the signal line 100. FIG. 7(C) shows theline input voltage V_(L) and the threshold voltage V_(TH) =0 which aresupplied to the comparator CP1, and FIG. 7(D) shows the output voltageV_(CO) of the comparator CP1. FIG. 7(E) shows the voltage V_(C) acrossthe capacitor C5. FIG. 7(F) shows the ON/OFF state of the switchingtransistor Tr1, and FIG. 7(G) shows the ON/OFF state of the transistorTr2. FIG. 7(H) shows the power sink input voltage V_(I) supplied to thepower sink 118, FIG. 7(I) shows the power sink output voltage V_(O)output from the power sink 118, and FIG. 7(J) shows the voltage V_(SW)output from the CPU 5. FIG. 7(K) shows the ON/OFF state of the hookswitch HS1, and FIG. 7(L) shows the ON/OFF state of the sub hook switchHS2.

In FIG. 8, the ISDN supplies the signal Info0 to the ISDN terminal andthe ISDN terminal supplies the signal Info0 to the ISDN. In this state,the power is OFF. Then, the power turns ON and the ISDN terminalsupplies the signal Info1 to the ISDN, and the ISDN supplies the signalInfo2 to the ISDN terminal. The ISDN terminal supplies the signal Info3in response to the signal Info2. Thereafter, the ISDN supplies thesignal Info4 to the ISDN terminal and the ISDN terminal makes a set upwith respect to the ISDN. The ISDN makes a call proceeding, an alertingand a connect, and a communication state is reached between the ISDN andthe ISDN terminal.

3) Operation When Disconnected From ISDN Side:

In this case, an on-hook is made after a disconnect signal is receivedfrom the ISDN side. In this state, the hook switch HS1 is OFF and thesub hook switch HS2 is ON.

However, because the state maintaining circuit 1 is ON, the transistorsTr1 and Tr2 will not turn OFF and the power is supplied to the internalcircuits. Thereafter, the layer 1 becomes inactive and no line isdetected on the signal line.

When the CPU 5 ends all processes and sends a low-level switch offsignal (voltage V_(SW)) to the photocoupler PC1, the photocoupler PC1turns ON. In this state, the voltage V_(C) stored in the capacitor C5 isdischarged to the ground level (GND) with a time constant determined bythe resistor r12 and the capacitor C5. As a result, the transistors Tr2and Tr1 successively turn OFF and the power supply to the internalcircuits is stopped. Therefore, it is possible to suppress the powerconsumption during states other than the communication state.

FIG. 9 is a timing chart showing the various timings when thedisconnection is made from the ISDN side, and FIG. 10 shows a sequenceof operations which are carried out when the disconnection is made fromthe ISDN side.

FIG. 9(A) shows the signal received on the signal line 101 and FIG. 9(B)shows the signal transmitted on the signal line 100. FIG. 9(C) shows theline input voltage V_(L) and the threshold voltage V_(TH) which aresupplied to the comparator CP1, and FIG. 9(D) shows the output voltageV_(CO) of the comparator CP1. FIG. 9(E) shows the voltage V_(C) acrossthe capacitor C5. FIG. 9(F) shows the ON/OFF state of the switchingtransistor Tr1, and FIG. 9(G) shows the ON/OFF state of the transistorTr2. FIG. 9(H) shows the power sink input voltage V_(I) supplied to thepower sink 118, FIG. 9(I) shows the power sink output voltage V_(O)output from the power sink 118, and FIG. 9(J) shows the voltage V_(SW)output from the CP1; 5. FIG. 9(K) shows the ON/OFF state of thephotocoupler PC1, and FIG. 9(L) shows the ON/OFF state of the hookswitch HS1.

In FIG. 10, the power is ON in the communication state. The ISDNdisconnects with respect to the ISDN terminal, and the ISDN terminalreleases with respect to the ISDN. Hence, the ISDN makes a releasecomplete and sends the signal Info0. The power is turned OFF in responseto the signal Info0 and the ISDN terminal sends the signal Info0 to theISDN.

4) Operation When Disconnected From ISDN Terminal Side:

In this case, the hook switch HS1 turns OFF and the sub hook switch HS2turns ON responsive to an on-hook. However, the power supply is madebecause the state maintaining circuit 1 is ON and operates. Thereafter,the operation is similar to the case described above when thedisconnection is made from the ISDN side. When the layer 1 becomesinactive and the CPU 5 ends all processes, the CPU 5 sends a low-levelswitch off signal (voltage V_(SW)) to the photocoupler PC1 and thephotocoupler PC1 turns ON. In this state, the voltage V_(C) stored inthe capacitor C5 is discharged to the ground level (GND) with a timeconstant determined by the resistor r12 and the capacitor C5. As aresult, the transistors Tr2 and Tr1 successively turn OFF and the powersupply to the internal circuits is stopped. Therefore, it is possible tosuppress the power consumption during states other than thecommunication state.

FIG. 11 is a timing chart showing the various timings when thedisconnection is made from the ISDN terminal side, and FIG. 12 shows asequence of operations which are carried out when the disconnection ismade from the ISDN terminal side.

FIG. 11(A) shows the signal received on the signal line 101 and FIG.11(B) shows the signal transmitted on the signal line 100. FIG. 11(C)shows the line input voltage V_(L) and the threshold voltage V_(TH)which are supplied to the comparator CP1, and FIG. 11(D) shows theoutput voltage V_(CO) of the comparator CP1. FIG. 11(E) shows thevoltage V_(C) across the capacitor C5. FIG. 11(F) shows the ON/OFF stateof the switching transistor Tr1, and FIG. 11(G) shows the ON/OFF stateof the transistor Tr2. FIG. 11(H) shows the power sink input voltageV_(I) supplied to the power sink 118, FIG. 11(I) shows the power sinkoutput voltage V_(O) output from the power sink 118, and FIG. 11(J)shows the voltage V_(SW) output from the CPU 5. FIG. 11(K) shows theON/OFF state of the photocoupler PC1, and FIG. 11(L) shows the ON/OFFstate of the hook switch HS1.

In FIG. 12, the power is ON in the communication state. The ISDNterminal disconnects with respect to the ISDN, and the ISDN releaseswith respect to the ISDN terminal. Hence, the ISDN terminal makes arelease complete. The ISDN sends the signal Info0, and the power isturned OFF in response to the signal Info0. The ISDN terminal sends thesignal Info0 to the ISDN.

Therefore, the power consumption is kept down to a minimum when nosignal is received on the signal line, and the power is supplied to theinternal circuits only when a signal is received on the signal line.

FIG. 13 shows the structure of one frame of layer 1. In FIG. 13, Fdenotes a framing bit, L denotes a DC balancing bit, D denotes aD-channel bit, E denotes a D-echo-channel bit, FA denotes an auxiliaryframing bit, N denotes a bit which is set to a binary value N=FA, B1denotes a bit within the B-channel 1, B2 denotes a bit within theB-channel 2, A denotes a bit used for activation, S denotes a spare bitfor future standardization, and M denotes a multiframing bit. Inaddition, a dot indicates a part of a frame which is independently DCbalanced. The FA bit in the direction TE to NT1 (or NT2) is used as a Qbit for every 5 frames when the Q-channel capability is provided. The2-bit offset is obtained at the output point of TE. A correspondingoffset at the NT1 (or NT2) may vary considerably depending on the delayand connection of the interface cable.

FIG. 14 is a diagram for explaining the definition of the signal Info.

Next, a description will be given of another embodiment of the statemaintaining circuit 2 shown in FIG. 3, by referring to FIG. 15. In FIG.15, those parts which are basically the same as those correspondingparts in FIG. 4 are designated by the same reference numerals, and adescription thereof will be omitted.

In FIG. 15, the state maintaining circuit 2 includes transistors Tr2 andTr3, resistors r9 through r12 and r20, a capacitor C10 and a diode D10.A node NX1 is connected to the photocoupler PC1 of the cancel circuit 4.A node NX2 is connected to the resistor r8 of the line signal detectioncircuit 1. In this embodiment, when the line signal detection circuit 1detects the signal on the signal line 101 and the comparator CP1 outputsthe voltage corresponding to the high level, the capacitor C10 ischarged and the transistor Tr3 turns ON. As a result, a current flowsthrough the resistor r9 and the transistor Tr3, and the transistor Tr1turns ON. The power sink input voltage V_(I) causes a current to flowthrough the resistors r11 and r20, and the transistor Tr2 turns ON. Whenthe transistor Tr2 is ON, a current flows through the resistors r9 andr10, the transistor Tr2 and the diode D10. Thus, the transistor Tr1turns ON, and since the power sink input voltage V_(I) becomes highlevel via the transistor Tr1, the current flows through the resistorsr11 and r20 and the transistor Tr2 remains ON even after the outputvoltage V_(CO) of the comparator CP1 falls to the low level and thetransistor Tr3 turns OFF. The transistor Tr2 turns OFF when thephotocoupler PC1 of the cancel circuit 4 turns ON and the potential atthe node NX1 decreases. As a result, the state maintaining operation ofthe state maintaining circuit 2 is disabled by the cancel circuit 4.

Next, a description will be given of a second embodiment of thecommunication terminal according to the present invention, by referringto FIG. 16. In FIG. 16, those parts which are basically the same asthose corresponding parts in FIG. 3 are designated by the same referencenumerals, and a description thereof will be omitted.

In this embodiment, the line signal detection circuit 1 is connected toa line 205 at inputs of the drivers 102 instead of being connected tothe line 101. In other words, the line signal detection circuit 1 iscoupled to the primary windings of the transformer T2 in the firstembodiment, but the line signal detection circuit 1 is coupled to thesecondary windings of the transformer T2 in this second embodiment.

In addition, in this second embodiment, the power sink 118 is coupled tothe lines 100 and 101 via the diode bridge 116, and the switch SW, thestate maintaining circuit 2 and the cancel circuit 4 are coupled to thepower input line 219 on the output side of the power sink 118. The samecircuits shown in FIG. 4 may be used in this embodiment, although thecircuit constants are selected differently between the first and secondembodiments. In addition, the operation of this embodiment is basicallythe same as that of the first embodiment described in conjunction withFIGS. 3 and 4. However, in this embodiment, the comparator CP1 of theline signal detection circuit 1 is not driven by the voltage of 40 V andis driven by a voltage of 5 V. As a result, an inexpensive comparatormay be used for the comparator CP1 when compared to the firstembodiment. Furthermore, the transistors need not operate at the largevoltage of 40 V, and it is possible to use inexpensive transistors whichare not bulky. The effects of this second embodiment are the same asthose obtainable in the first embodiment.

In the described embodiments, the present invention is applied to theISDN terminal. However, it is of course possible to apply the presentinvention to any kind of communication terminal which is coupled to acommunication network via a switching system.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A communication terminal which is coupled to acommunication network via transmitting and receiving signal lines and aswitching system, said communication terminal comprising:a power inputline coupled to the transmitting and receiving signal lines; a powersink for supplying power; switching means coupled between said powerinput line and said power sink; a hook switch coupled between said powerinput line and said power sink in parallel to said switching means;detection circuit means coupled to the receiving signal line fordetecting a signal on the receiving signal line; state maintainingcircuit means coupled to said switching means and said detection circuitmeans for closing said switching means and for maintaining saidswitching means in a closed state by carrying out a state maintainingoperation when a signal is detected by said detection circuit means; andprohibit circuit means coupled to said detection circuit means forprohibiting a signal detection of said detection circuit means.
 2. Thecommunication terminal as claimed in claim 1, wherein said prohibitcircuit means is coupled to said hook switch and prohibits the signaldetection of said detection circuit means in response to a closingoperation of said hook switch.
 3. The communication terminal as claimedin claim 2, wherein said detection circuit means includes amplifiercircuit means for amplifying the signal on the receiving signal line andcomparator means for detecting the signal on the receiving signal lineby comparing the amplified signal with a threshold voltage.
 4. Thecommunication terminal as claimed in claim 3, wherein said prohibitcircuit means includes a sub hook switch which is linked to said hookswitch and cuts off a supply of power to said comparator means whenopen, said sub hook switch opening when said hook switch closes.
 5. Thecommunication terminal as claimed in claim 1, which further comprisescontrol means including means for controlling operations of saidcommunication terminal and means for outputting a switch off signal, andcancel circuit means coupled to said control means for supplying asignal for cancelling the state maintaining operation of said statemaintaining circuit means responsive to the switch off signal from saidcontrol means when said hook switch is open.
 6. The communicationterminal as claimed in claim 1, which further comprises transformershaving primary windings coupled to said transmitting and receivingsignal lines, drivers and receivers respectively coupled to saidtransmitting and receiving signal lines via to secondary windings ofsaid transformers, and line interface means coupled to said drivers andreceivers, said detection circuit means being connected to the primarywindings of the transformer which is coupled to the receiving line. 7.The communication terminal as claimed in claim 1, wherein said detectioncircuit means includes amplifier circuit means for amplifying the signalon the receiving signal line and comparator means for detecting thesignal on the receiving signal line by comparing the amplified signalwith a threshold voltage.
 8. The communication terminal as claimed inclaim 7, wherein said amplifier circuit means amplifies the signal onthe receiving signal line with an amplification of approximately two. 9.The communication terminal as claimed in claim 1, which furthercomprises diode bridge circuit means coupled between said power inputline and the transmitting and receiving signal lines.
 10. Thecommunication terminal as claimed in claim 1, wherein said statemaintaining circuit means includes first switching means coupled to saidpower input line for supplying power to said power sink by turning ONwhen said detection circuit means detects the signal on the receivingsignal line and second switching means coupled to said first switchingmeans for maintaining said first switching circuit ON even after saiddetection circuit means no longer detects the signal on the receivingsignal line.
 11. The communication terminal as claimed in claim 10,wherein said prohibit circuit means is coupled to said hook switch andprohibits the signal detection of said detection circuit means inresponse to a closing operation of said hook switch when saidcommunication terminal is called.
 12. The communication terminal asclaimed in claim 10, wherein said prohibit circuit means is coupled tosaid hook switch and prohibits the signal detection of said detectioncircuit means in response to a closing operation of said hook switchwhen said communication terminal is calling.
 13. The communicationterminal as claimed in claim 10, which further comprises control meansincluding means for controlling operations of said communicationterminal and means for outputting a switch off signal when communicationends, and cancel circuit means coupled to said control means forsupplying a signal for cancelling the state maintaining operation ofsaid second switching means of said state maintaining circuit meansresponsive to the switch off signal from said control means when saidhook switch is open after a disconnection is made from the communicationnetwork.
 14. The communication terminal as claimed in claim 10, whichfurther comprises control means including means for controllingoperations of said communication terminal and means for outputting aswitch off signal when communication ends, and cancel circuit meanscoupled to said control means for supplying a signal for cancelling thestate maintaining operation of said second switching means of said statemaintaining circuit means responsive to the switch off signal from saidcontrol means when said hook switch is open after a disconnection ismade from said communication terminal.
 15. The communication terminal asclaimed in claim 1, wherein the communication network is an integratedservices digital network.
 16. A communication terminal which is coupledto a communication network via transmitting and receiving signal linesand a switching system, said communication terminal comprising:a powerinput line coupled to the transmitting and receiving signal lines; apower sink coupled to said power input line for supplying power from anoutput side thereof; switching means coupled to the output side of saidpower sink; a hook switch coupled to the output side of said power sinkin parallel to said switching means; detection circuit means coupled tothe receiving signal line for detecting a signal on the receiving signalline; state maintaining circuit means coupled to said switching meansand said detection circuit means for closing said switching means andfor maintaining said switching means in a closed state by carrying out astate maintaining operation when a signal is detected by said detectioncircuit means; and prohibit circuit means coupled to said detectioncircuit means for prohibiting a signal detection of said detectioncircuit means.
 17. The communication terminal as claimed in claim 16,wherein said prohibit circuit means is coupled to said hook switch andprohibits the signal detection of said detection circuit means inresponse to a closing operation of said hook switch.
 18. Thecommunication terminal as claimed in claim 17, wherein said detectioncircuit means includes amplifier circuit means for amplifying the signalon the receiving signal line and comparator means for detecting thesignal on the receiving signal line by comparing the amplified signalwith a threshold voltage.
 19. The communication terminal as claimed inclaim 18, wherein said prohibit circuit means includes a sub hook switchwhich is linked to said hook switch and cuts off a supply of power tosaid comparator means when open, said sub hook switch opening when saidhook switch closes.
 20. The communication terminal as claimed in claim16, which further comprises control means including means forcontrolling operations of said communication terminal and means foroutputting a switch off signal, and cancel circuit means coupled to saidcontrol means for supplying a signal for cancelling the statemaintaining operation of said state maintaining circuit means responsiveto the switch off signal from said control means when said hook switchis open.
 21. The communication terminal as claimed in claim 16, whichfurther comprises transformers having primary windings coupled to saidtransmitting and receiving signal lines, drivers and receiversrespectively coupled to said transmitting and receiving signal lines viato secondary windings of said transformers, and line interface meanscoupled to said drivers and receivers, said detection circuit meansbeing coupled to the secondary windings of the transformer which iscoupled to the receiving line.
 22. The communication terminal as claimedin claim 21, wherein said detection circuit means is connected to inputsides of said receivers.
 23. The communication terminal as claimed inclaim 16, wherein said detection circuit means includes amplifiercircuit means for amplifying the signal on the receiving signal line andcomparator means for detecting the signal on the receiving signal lineby comparing the amplified signal with a threshold voltage.
 24. Thecommunication terminal as claimed in claim 23, wherein said amplifiercircuit means amplifies the signal on the receiving signal line with anamplification of approximately two.
 25. The communication terminal asclaimed in claim 16, which further comprises diode bridge circuit meanscoupled to said power input line between said power sink and thetransmitting and receiving signal lines.
 26. The communication terminalas claimed in claim 16, wherein said state maintaining circuit meansincludes first switching means coupled to said power input line forenabling a power supply from said power sink by turning ON when saiddetection circuit means detects the signal on the receiving signal lineand second switching means coupled to said first switching means formaintaining said first switching circuit ON even after said detectioncircuit means no longer detects the signal on the receiving signal line.27. The communication terminal as claimed in claim 26, wherein saidprohibit circuit means is coupled to said hook switch and prohibits thesignal detection of said detection circuit means in response to aclosing operation of said hook switch when said communication terminalis called.
 28. The communication terminal as claimed in claim 26,wherein said prohibit circuit means is coupled to said hook switch andprohibits the signal detection of said detection circuit means inresponse to a closing operation of said hook switch when saidcommunication terminal is calling.
 29. The communication terminal asclaimed in claim 26, which further comprises control means includingmeans for controlling operations of said communication terminal andmeans for outputting a switch off signal when communication ends, andcancel circuit means coupled to said control means for supplying asignal for cancelling the state maintaining operation of said secondswitching means of said state maintaining circuit means responsive tothe switch off signal from said control means when said hook switch isopen after a disconnection is made from the communication network. 30.The communication terminal as claimed in claim 26, which furthercomprises control means including means for controlling operations ofsaid communication terminal and means for outputting a switch off signalwhen communication ends, and cancel circuit means coupled to saidcontrol means for supplying a signal for cancelling the statemaintaining operation of said second switching means of said statemaintaining circuit means responsive to the switch off signal from saidcontrol means when said hook switch is open after a disconnection ismade from said communication terminal.
 31. The communication terminal asclaimed in claim 16, wherein the communication network is an integratedservices digital network.